The invention concerns a method of operation of a device for determining the position of a gas/liquid interface of a liquefied gas in a cryogenic tank, the device having at least one self-heated resistance temperature detector mentioned here down also like a temperature sensor being mounted inside the cryogenic tank by a support, the detector being connected to a current pulse generator, the device further having means to read out the temperature of the detector, the readout depending on the electric resistance of said detector, the method of operation comprising the application of at least one current from the pulse generator to the detector, the power and duration of this heating pulse being sufficient for overheating the detector at the end of this heating pulse to a temperature Theated above at least the temperature of its environment Tenv.
A method as described above is known from Reference [9].
In general, the present invention relates to measuring the liquid level or position of a gas/liquid interface in a tank of cryogenic liquid in a wide temperature range and, in particular, the method allows to locate the surface of a pumped bath of both normal and superfluid helium.
It is often necessary to locate the surface or the liquid/vapor interface of a liquid inside a closed vessel. This also includes a task to determine the level of the liquid inside a pumped bath of He I or He II accurately inside cryogenic tank. A number of reports on cryogenic level meters have been published utilizing superconductors, resistors, capacitors, transmission lines, and other elements (as described in the review of Reference [1] and the references cited therein).
The idea to use temperature sensor as a level monitor is based on the well-established fact that the thermal contact between the sensor and the liquid is better than the thermal contact between the sensor and surrounding gas. The sensitivity of such an approach ΔR/R1(I) where R1(I) is resistance of sensor in liquid, Rg(I) is the resistance of the sensor gas and ΔR=R1−Rg is a function of the current I. At small currents one should see virtually no difference between sensor-in-liquid and sensor-in-gas states. As the power is increased, |ΔR| increases until the Leidenfrost point is reached (at this point the boiling of the helium around the sensor becomes so rapid that the resistor is surrounded with an envelope of vapor). As more power is dissipated, the distinction between being in liquid and in vapor disappears and ΔR approaches to zero again. For example, the study of Allen-Bradley 0.125 W carbon resistors as sensors to probe helium level at 4.2 K had revealed that the maximum sensitivity can reach up to 0.35 and the corresponding power at maximum sensitivity is a weak function of the nominal resistance and is of about 0.1 W (see Reference [2]).
At 4.2 K one can reduce the current until the relative change ΔR/R1(I) is still well above the noise/fluctuation level (see References [3], [4]). Unfortunately this approach is not valid if the sensor is intended to be used in a wide temperature range.
In the limit of small currents, the temperature of the overheated sensor immersed in the gas at low temperature can be lower than the temperature of the overheated sensor immersed in liquid at high temperature, especially below superfluid transition. The creeping superfluid film usually ensures that everything inside the cryostat is maintained at the same temperature as that of the liquid for a distance of several centimeters above its surface. An additional power up to tens of mW (depending on the device construction) is required to prevent the film creeping.
Another disadvantage of the statically overheated approach is the absence of a simple liquid vs. vapor criterion. It is easy to determine the level while moving the sensor from the top of the cryostat to the bottom (dip-stick type of measurements) until the R(h) (where h is the height of the stick) dependence will show a characteristic discontinuity.
This approach requires moving parts and sliding sealing. For the fixed positioned sensor (when the helium level passes through the fixed sensor location), in the limit of small currents, the temperature of the overheated sensor surrounded by the gas at low temperature might happened to be lower than the temperature of the overheated sensor immersed in liquid at high temperature. As a result, one has to increase current, but in this case due to the approaching to the Leidenfrost regime it becomes difficult to distinguish between sensor in liquid and sensor in gas states even for the single temperature.
One of the possible solutions is to burn the film with a separate heater. An elegant thermal device was suggested in Reference [5] and improved by Reference [6]. Basically it consists of a separate heater and a sensor. The sensor is connected to the heater via weak thermal link. The power of the heater is adjusted strong enough to burn out the superfluid film. The status of the device (in or out of the liquid) is determined as follows. A digital voltmeter is connected parallel to the sensor; the current through the sensor is set to a small value of 10 μA via a simplified current source. The heater is periodically switched on and off via connecting/disconnecting it to a variable power supply. In liquid, the thermal contact of the sensor to the surrounding helium is much stronger than to the heater. The sensor is strongly anchored to the helium bath. As a result the voltage at the sensor Usensor is practically insensitive to the status of the heater (on or off). In gas, the thermal contact of the sensor with the environment is reduced and the temperature of the sensor will follow the temperature of the heater. The output voltage will depend on the status of the heater (on or off) or fluctuate. Thus the fluctuating voltage can be used as a simple criterion of the sensor in gas state independent on the helium bath temperature (see Reference [6]).
The improved device (see Reference [6]) consists of pair of 51 Ohm 0.125 W Allen Bradley carbon resistors mounted on a 4 mm diameter stainless steel tube and fixed by GE7031 varnish. The length of the unit is less than 12 mm. The lower resistor is used as a heater while the upper serves as a temperature sensor. The burning of the super-thermal film requires some time and energy. If the device is placed just above the surface of superfluid helium, the heater voltage of 2.5 V was enough to observe the resistance jump down in tburn=2−3 s from 3.8 kOhm to 850 Ohm. In other words, the time response of this particular thermal device is rather slow—the duration of measuring pulse should be at least 3 s, while the thermal time constant of the sensor itself is in ms range (see Reference [7]). The energy consumption during the measuring pulse was estimated as ˜0.1 J or the average power load to the heater during the measuring is at least 33 mW (see Reference [6]).
An alternative to the statically overheated approach is to use standing along self-heated detector in a pulsed mode to simplify the sensor construction and to reduce the average power dissipation of radio resistor based level indicators (see Reference [8]). In this setup, authors analyze the voltage profile across the sensor which is characteristic for helium gas phase and liquid phase.
For the silicon diode based sensors this approach was proposed in Reference [9] to measure the level of liquid nitrogen in a cryogenic tank. A relatively large (e.g. 30 mA) current is pulsed (for several seconds) through a forward-biased silicon diode. The thermal transients of the sensor (its thermal responses to the switching on and to the switching off) are studied. The temperature of the sensor is probed by request via a separate procedure: the current for a short time (few microseconds) is switched to a smaller value where the voltage across the diode is a known function of temperature. Resulting temperature transients are recorded and analyzed by a special data acquisition/analysis system to extract times required to overheat and to cool down the diode. These time constants are compared with typical values (found in a separate set of “learning” measurements) and, based on this comparison, the status of the sensor (in liquid or in gas) is assigned.
A similar approach was proposed in Reference [10] to probe the fuel level in aircrafts. The method of sensing the presence of a fluid includes the steps of energizing a temperature-dependent resistance to produce alternate cycles of warming and cooling and monitoring the rate at which the sensor warms and/or cools. These rates are characteristic of a resistance affected or unaffected by the presence of a fluid.
As for disadvantages of the prior art realization of the pulsed mode detectors one can count rather sophisticated hardware and non-trivial computer based computation procedure:
Several (at least two) temperature (or resistance) measurements must be taken during warming up and/or cooling down circle/circles,
based on these measurements, a computation procedure is involved to calculate characteristic warming up and/or cooling down rates,
the conclusion on the detector status is made based on comparison of these computed values with predefined (in a separate experiment) table values. These table values might depend on the temperature of surrounding, that's why methods in References [9], [10] imply direct or indirect knowledge of the temperature of the fluid.
It is a principal object of the invention to provide a robust apparatus and alternative and effective method for sensing and detection the level of a liquid, in particular of cryogenic liquids.
Another object of the invention is to provide a helium level indicator capable to detect the position of the helium liquid/gas interface in temperature range 1.0 to 4.2 K; having a low power consumption (less than 10 mW); characterized by a short response (less than 250 ms); providing a level position resolution better than 1 mm; based on robust liquid vs. gas criterion valid in the whole temperature range; having a simple and reliable thermal device construction.